1. Technical Field
This application relates to computer storage devices, and more particularly to cache used in connection with computer storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may nor correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
Performance of a storage system may be improved by using a cache. In the case of a disk drive system, the cache may be implemented using a block of semiconductor memory that has a lower data access time than the disk drive. Data that is accessed is advantageously moved from the disk drives to the cache so that the second and subsequent accesses to the data may be made to the cache rather than to the disk drives. Data that has not been accessed recently may be removed from the cache to make room for new data. Often such cache accesses are transparent to the host system requesting the data.
In many instances, increasing the amount of cache memory increases system performance. However, due to the expense of extra main memory, there is a point of diminishing returns where the cost of additional main memory for cache exceeds the value of the benefit. On the other hand, if additional memory for cache could be obtained at little or no cost, then it may be beneficial to add more cache memory.